FRAME RELAY NETWORK. High speed networks

FRAME RELAY NETWORK

1. 1
UNIT 1
HIGH SPEED NETWORKS

2. 2
INTRODUCTION
Packet-Switching Networks
–Switching Technique
–Routing
–X.25
Frame Relay Networks
–Architecture
–User Data Transfer
–Call Control

3. 3
Packet-Switching Networks
Basic technology the same as in the 1970s
One of the few effective technologies for long
distance data communications
Frame relay and ATM are variants of packet-
switching
Advantages:
–flexibility, resource sharing, robust, responsive
Disadvantages:
–Time delays in distributed network, overhead
penalties
–Need for routing and congestion control

4. 4
Circuit-Switching
Telecom network designed for voice
is an example for circuit switching
Network resources dedicated to one
call
Shortcomings when used for data:
–Inefficient (high idle time)
–Constant data rate

5. 5
Packet-Switching
Data transmitted in short blocks or
packets
Packet length < 1000 octets
Each packet contains user data plus
control info (routing)
Store and forward

6. 6
The Use of Packets

7. 7
Packet Switching: Datagram Approach

8. 8
Advantages over Circuit-Switching
Greater line efficiency (many packets can
go over shared link)
Data rate conversions
Non-blocking under heavy traffic (but
increased delays)

9. 9
Disadvantages relative to Circuit-
Switching
Packets incur additional delay with every
node they pass through
Jitter: variation in packet delay
Data overhead in every packet for routing
information, etc

10. 10
Figure 4.3 Simple Switching
Network

11. 11
Switching Technique
Large messages broken up into smaller packets
Datagram
–Each packet sent independently of the others
–No call setup
–More reliable (can route around failed nodes or
congestion)
Virtual circuit
–Fixed route established before any packets sent
–No need for routing decision for each packet at
each node

12. 12
Figure 4.4 Packet
Switching: Virtual-
Circuit Approach

13. 13
Routing
“Process of routing the packet from one node
to another node”
Node/trunk failure
Congestion

14. 14
X.25 Protocol
3 levels
Physical level (X.21) Provides physical
interface between the station and link attached with
node
Link level Reliable transfer of data (LAPB-Link
access protocol balanced, a subset of HDLC –High
level data link control)
Packet level provides virtual circuit service)d

15. 15
The Use of Virtual Circuits

16. 16
User Data and X.25 Protocol Control
Information

17. 17
Frame Relay Networks
Designed to eliminate much of the overhead
in X.25
Call control signaling on separate logical
connection from user data
Multiplexing/switching of logical
connections at layer 2 (not layer 3)
No hop-by-hop flow control and error control
Throughput an order of magnitude higher
than X.25

18. 18
Comparison of X.25 and Frame Relay
Protocol Stacks

19. 19
Virtual Circuits and Frame Relay Virtual
Connections

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Frame Relay Architecture
X.25 has 3 layers: physical, link, network
Frame Relay has 2 layers: physical and data
link (LAPF-Link access procedure for frame
mode bearer services )
LAPF core: minimal data link control
–Preservation of order for frames
–Small probability of frame loss
LAPF control: additional data link or network
layer end-to-end functions

21. 21
LAPF Core
Frame delimiting, alignment and
transparency
Frame multiplexing/demultiplexing
Inspection of frame for length constraints
Detection of transmission errors
Congestion control

22. 22
User Data Transfer
Data frames includes send and receive sequence
number
Sequence number is used to allow the receiver to
control the rate of incoming frame and to report
missing or damaged frames for retransmission
Send sequence number is used to number each
transmitted frame sequentially
Receiver sequence is used to provide (+) / (-)
acknowledgement to incoming frames

23. 23
Figure 4.9 LAPF-core
Formats

24. Flag:
Indicates the start and end of the frame
Frame check sequence(FCS):
Error detection on transmission side
Information field:
Carries user data / call control messages
Address field:
Carries DLCI(Data link connection identifier)
24

25. 25
Frame Relay Call Control
Frame relay supports multiple connections over a
single link
Data transfer involves:
–Establish logical connection and assign DLCI
–Exchange data frames
–Release logical connection

26. 26
Frame Relay Call Control
4 message types needed
SETUP
CONNECT
RELEASE
RELEASE COMPLETE